Editing Another User's Tasks folder

We would like to allow two users to edit the same task. However, assigning the task prevents everyone but the current owner from editing it. How can we delegate multiple owners for a task?

Microsoft Office Outlook items can have only one owner. However, if User A is the owner and User B has permission to edit items in User A's Tasks folder, User B can edit all of User A's tasks, including any task that User A now owns as a result of accepting a task assignment.